Wireless sensors can be implanted within the body and used to monitor physical conditions, such as pressure or temperature. For example, U.S. Pat. No. 6,111,520, U.S. Pat. No. 6,855,115 and U.S. Publication No. 2003/0136417, each of which is incorporated herein by reference, all describe wireless sensors that can be implanted within the body. These sensors can be used to monitor physical conditions within the heart or an abdominal aneurysm. An abdominal aortic aneurysm (AAA) is a dilatation and weakening of the abdominal aorta that can lead to aortic rupture and sudden death. In the case of a repaired abdominal aneurysm, a sensor can be used to monitor pressure within the aneurysm sac to determine whether the intervention is leaking. The standard treatment for AAAs employs the use of stent-grafts that are implanted via endovascular techniques. However, a significant problem that has emerged with these stent-grafts for AAAs is acute and late leaks of blood into the aneurysm's sac. Currently, following stent-graft implantation, patients are subjected to periodic evaluation via abdominal CT (Computed Tomography) with IV contrast to identify the potential presence of stent-graft leaks. This is an expensive, risky procedure that lacks appropriate sensitivity to detect small leaks.
Typically, the sensors utilize an inductive-capacitive (“LC”) resonant circuit with a variable capacitor. The capacitance of the circuit varies with the pressure of the environment in which the sensor is located and thus, the resonant frequency of the circuit varies as the pressure varies. Thus, the resonant frequency of the circuit can be used to calculate pressure.
Ideally, the resonant frequency is determined using a non-invasive procedure. A system and method for determining the resonant frequency of an implanted sensor are discussed in U.S. application Ser. No. 11/276,571 entitled “Communicating with an Implanted Wireless Sensor” filed Mar. 6, 2006 (the '571 application”). The signal from the sensor is weak relative to the signal used to energize the sensor, but is the same frequency and dissipates quickly. In one embodiment, the difference between the signals is on the order of 150 dB and the sensor signal is sampled approximately 35 nanoseconds after the energizing signal is turned off. In order to communicate with the sensor, the system uses a coupling loop and a cable assembly. Due to the unique characteristics of the transmitted and received signals the coupling loop and the cable assembly need to isolate the energizing signal and the sensor signal, support the necessary sampling speed, and support a relatively large bandwidth.
Some prior art coupling loops use switched capacitor banks to meet the bandwidth requirement, but there are disadvantages to using switched capacitor banks regardless of the type of switching mechanism used. There are reliability issues associated with mechanical relays and loss issues associated with solid-state switches. Thus, there is a need for a coupling loop that provides the required bandwidth, but does not use switched capacitor banks.
A reflection or resonance from another object in the vicinity of the sensor can cause the system to lock on a frequency that does not correspond to the resonant frequency of the sensor, i.e. generates a false lock. Optimizing the position of the coupling loop relative to the sensor maximizes the coupling between the sensor and the coupling loop and reduces the sensitivity to a false lock. The coupling is maximized when the sensor is centered within the coupling loop and the inductor coil within the sensor is approximately parallel to the coupling loop. For many sensors this is achieved when the flat side of the sensor is approximately parallel to a plane defined by the coupling loop.
Thus, there is a need for indicating to a physician or other user the relative positions of the coupling loop and the sensor so that the sensor and the coupling loop are placed in magnetic proximity. In order to properly position the coupling loop, the coupling loop and the cable assembly should be easy to manipulate, which requires a lightweight coupling loop of a reasonable size and a flexible, lightweight cable with a relatively small diameter.